Conventionally, a latent image that is electrically or magnetically formed by an electrophotographic image forming apparatus is developed with an electrophotographic toner (hereinafter, may be referred to simply as “toner”). For example, in electrophotograph, an electrostatic charge image (latent image) is formed on a photoconductor, and then the latent image is developed with a toner, thereby to form a toner image. Usually, the toner image is transferred to a transfer material such as paper, and then fixed on the transfer material such as paper. In the fixing step of fixing the toner image on a transfer sheet, thermal fixing methods such as a heating roller fixing method and a heating belt fixing method are commonly used because these methods are energy-efficient.
Recently, there are increasing demands from the market for image forming apparatuses of high speed and energy saving, and therefore a toner having excellent low temperature fixability and capable of providing high quality images is desired. As a method for achieving the low temperature fixability of the toner, there is a method of lowering the softening point of the binder resin contained in the toner. However, when the softening temperature of the binder resin is low, it becomes easier for a so-called offset (also referred to as hot offset hereinafter) to occur, in which part of a toner image is deposited onto a surface of a fixing member during fixing, and then transferred to photocopy paper. In addition to this, the heat resistant storage stability of the toner degrades, and therefore toner particles are fused to each other particularly in high temperature environments, which is so called blocking. Besides, also in the developing device, problems occur that the toner melts and adheres to the interior of the developing device and the carrier to contaminate them, or that it becomes easier for the surface of the photoconductor to be filmed with the toner.
As for the technique for solving the aforementioned problems, it has been known to use a crystalline resin as a binder resin of the toner. Because the crystalline resin has a characteristic of rapidly softening from its crystallized state when it reaches the melting point, it can greatly lower the fixing temperature of the toner while maintaining the heat resistant storage stability at the temperature equal to or lower than the melting point. That is, the crystalline resin can realize both of low temperature fixability and heat resistant storage stability at high levels. However, the crystalline resin, which has a melting point at which it expresses low temperature fixability, is soft and would easily undergo plastic deformation, although it has excellent toughness. Therefore, when the only measure taken is to use the crystalline resin as the binder resin, the toner would be very poor in the mechanical durability and would cause various problems such as deformation, agglomeration, and solidification of the toner in the image forming apparatus, contamination of the members in the apparatus by the toner, etc.
Hence, many toners in which a crystalline resin and a non-crystalline resin are used in combination have been conventionally proposed as toners in which a crystalline resin is used as the binder resin (see, e.g., PTL 1 to PTL 5). As compared with conventional toners made of only a non-crystalline resin, these toners are excellent in realizing both low temperature fixability and heat resistant storage stability. However, if the crystalline resin is exposed above the surface of the toner, agglomerates of toner particles would occur from stirring stress in the developing device, which might cause transfer voids. Therefore, the proposed techniques have not been able to fully take advantage of the crystalline resin, because the additive amount of the crystalline resin has to be suppressed.
Further, many toners have been proposed that use a resin in which a crystalline segment and a non-crystalline segment are chemically bonded. For example, toners that use as a binder resin, a resin in which crystalline polyester and polyurethane are bonded are proposed (see, e.g., PTL 6 and PTL 7). Further, toners that use a resin in which crystalline polyester and amorphous vinyl polymer are bonded are proposed (see, e.g., PTL 8). Furthermore, toners that use as a binder resin, a resin in which crystalline polyester and non-crystalline polyester are bonded are proposed (see, e.g., PTL 9 to PTL 11).
Moreover, there are proposed a technique of adding inorganic fine particles to a binder resin, of which main component is a crystalline resin (see, e.g., PTL 12), and a technique for a toner that uses a crystalline resin having a cross-linked structure formed by an unsaturated bond containing a sulfonic acid group (see, e.g., PTL 13).
All of these proposed techniques are excellent in realizing both low temperature fixability and heat resistant storage stability, but have failed in fundamentally curing the soft characteristic attributed to the crystalline segment, and have not been able to solve the problems concerning the mechanical durability of the toner.
Furthermore, one major problem of a toner using a crystalline resin is the friction resistance of an image. After the toner has melted on a fixing medium by thermal fixation, it takes time for the crystalline resin in the toner to recrystallize, and hence the toner cannot rapidly restore its hardness on the surface of the image. Therefore, the toner would generate scars on the surface of the image or change the glossiness of the image due to contact and sliding friction with a sheet discharging roller, a conveying member, etc. in the sheet discharging step after the fixation.
Therefore, currently, a toner is demanded that can realize both of low temperature fixability and heat resistant storage stability at high levels, prevents transfer voids due to occurrence of agglomeration of toner particles in the developing device, and has excellent friction resistance.